We study visual attention because of its importance in visual perception. Behavioral paradigms, such as change blindness, have shown us that while we think we perceive the whole visual world, we only take away information about regions or objects that we have attended. Because visual attention is a foundation of visual perception, it underlies most of our interactions with the perceived world - both our physical interactions and more intellectual interactions, such as learning and memory. Thus, increasing our understanding of the mechanisms underlying the guidance of attention is critical in allowing us to gain a deeper insight into how the brain makes decisions based on both external and cognitive inputs and, in the long run, insight into the mechanisms underlying visual perception itself. In this study, we test the hypothesis that the lateral intraparietal area (LIP) acts as a priority map - a map of the visual world that is used to guide the allocation of attention. The theory is that attention is allocated to the location on the map with the greatest activity. We have hypothesized that this map is used to guide both peripheral (covert) attention and eye movements (overt attention). In aim 1, we will test this hypothesis by comparing the activity in LIP and visual area V4 under conditions in which covert attention is spread, focused or biased to a particular location. We predict that activity in V4 will be modulated in a way that is directly related to the spatial distribution of activity in LIP - a peak of activity in LIP will produce strong attentional modulation in V4. We will further test this by stimulating LIP and showing predictable modulation in V4 activity. In aim 2, we will test a prediction made by our model of the system, namely that once an object has been looked at, it is suppressed on the map so that the focus of gaze (ie. overt attention) doesn't just bounce between the two highest points on the map. We will test this by examining the activity in LIP to an identical stimulus under conditions in which it has or has not been looked at previously. We expect that the response will be significantly lower in the case in which the stimulus has already been seen. We will then test whether this reduction in activity is important to the behavior by stimulating LIP during the task. We expect that this will result in more eye movements being made to the visual stimulus at the stimulated location than to the same stimulus in trials in which stimulation does not occur. These experiments are aimed at understanding the role that LIP plays in the allocation of attention and the results may be used to fine-tune our model of how attention is allocated. PUBLIC HEALTH RELEVANCE The results from this study help us understand the way the brain decides what is worth paying attention to. Given the importance of visual attention in everyday life and the deficits seen in patients with parietal lesions or attention deficit disorders, a greater understanding of this mechanism may aid in the development of pharmacological or behavioral methods to combat these problems.